Re: V6 evaluation inside Table and plot
- To: mathgroup at smc.vnet.net
- Subject: [mg76673] Re: V6 evaluation inside Table and plot
- From: dh <dh at metrohm.ch>
- Date: Fri, 25 May 2007 06:46:00 -0400 (EDT)
- References: <f33onq$l4v$1@smc.vnet.net>
Hi Gerry,
this is an ugly trap you encoutered. Consider: Attributes[Table] and you
will see that Table has the Attribute HoldAll. That means f'' is not
evaluated before beeing fed to Table. Now Table seems to have a small
brain damage, because it does the derivative wrong. You can see this from:
Table[f''[y], {y, 3, 3, 1}] // Trace
why this happens I can not tell, but I assume there is some confusion
between variables local to Function and Table. Please inform Wolfram
about this bug and let me know the answer.
hope this helps,Daniel
Gerry Flanagan wrote:
> Define a simple function
>
> In[7]:= f = Function[{x}, x^3]
>
> Out[7]= Function[{x}, x^3]
>
> Taking two derivatives works as it should
>
> In[2]:= f''[x]
>
> I was converting some packages to work with V6, and immediately ran into
> some evaluation issues I don't understand. It looks like forms like
> f''[x] (derivative) are evaluated differently inside of Table and Plot
> than outside.
>
> Out[2]= 6 x
>
> But this does not?
>
> In[20]:= Table[f''[x], {x, 0, 1, .2}]
>
> Out[20]= {0, 0, 0, 0, 0, 0}
>
> Strangely, this works
>
> In[4]:= Table[f'[x], {x, 0, 1, .2}]
>
> Out[4]= {0., 0.12, 0.48, 1.08, 1.92, 3.}
>
> Determining the derivative outside the Table makes it work again.
>
> In[12]:= g = f'';
>
> In[21]:= tmp = Table[g[x], {x, 0, 1, .2}]
>
> Out[21]= {0., 1.2, 2.4, 3.6, 4.8, 6.}
>
> Gerry Flanagan
>